The invention relates to a so-called skull crucible for melting or refining glasses or glass ceramics.
Troughs made of refractory material are strongly attacked by glasses with high melting points above 1650xc2x0 C., so that the dwell times become uneconomical and the produced glasses are full of stones, knots and streaks from the trough material.
At temperatures above 1650xc2x0 C., the use of an additional electric heating is strongly impaired because the corrosion of the electrodes such as Mo-electrodes increases strongly and the glasses are strongly colored by the impurities.
Aggressive glasses as are required for a number of optical applications for example strongly attack the ceramic refractory materials during the melting, and especially melt-down, even at lower temperatures. The strong attack on the trough allows neither an economic melting concerning the service life of the troughs, nor a precise adherence to the composition and, in connection with this, the required properties. That is why many of these glasses are molten in platinum troughs. A number of the aggressive glasses cannot be molten even in platinum crucibles, because they attack platinum and the dissolved platinum oxide colors the glass or the platinum oxide is reduced to platinum metal during the further processes and causes disturbances as platinum particles.
In high-purity glasses as are used in fiber optics for example, even very few ppb of coloring oxides which are introduced by the melting process can be disturbing.
The heating of glass by means of high frequency offers the possibility to couple the energy directly into the glass. Impurities by electrode corrosion can thus be prevented. The U.S. Pat. No. 4,780,121 describes a high-frequency heated ceramic refining crucible in which soda-lime-silica glass is refined at temperatures of between 1150xc2x0 C. and 1450xc2x0 C. The disadvantage of this method is that the refractory material is still very strongly attacked by the glasses at temperatures over 1700xc2x0 C.
As a result of the direct energy induction into the glass, the glasses can also be heated to temperatures above 1650xc2x0 C. When using ceramic crucible or trough materials, the temperature on the inner wall of the crucible should not exceed 1650xc2x0 C. In order to maintain this temperature it is necessary that the temperature gradient in the crucible wall must become increasingly steeper with rising temperatures, i.e. the crucible wall must become increasingly thinner and the cooling of the outer wall must become increasingly more intense. The cooling of the outer wall by natural convection as described in the U.S. Pat. No. 4,780,121 is limited within narrow margins since arc-overs occur between the crucible wall and the coil by the heated air. Higher melting temperatures can be achieved when the ceramic crucible is cooled by water-cooled copper pipes.
In a number of patent specifications (U.S. Pat. No. 3,461,215, DE 2 033 074, EP 0 119 877 B1, DE 3 316 546 C1) skull crucibles are described in which the ceramic inner crucible is omitted entirely. Melting temperatures of up to 3000xc2x0 C. are reached. In literature, continuously operating skull crucibles for melting radioactive materials are described. By using the skull crucible, it is possible to avoid obtaining radioactively contaminated trough material. No requirements are placed on the molten glass concerning the bubble quality.
DE 33 16 547 C2 describes a cold crucible for the melting of non-metallic organic compounds. A superstructure is placed on the upper crucible edge which consists of oxide ceramics. This superstructure is of cylindrical shape. It is used for reducing heat losses.
The disadvantage of all skull crucible systems as described in literature and the patents is that the water-cooled components reach into the gas space above the melt surface. A number of substantial problems are linked to this:
1. The melt surface is cooled by heat dissipation and the water-cooled skull crucible walls. This leads to a significant temperature gradient from the center to the surface of the melt. This is disadvantageous for the application as a refining unit, because the bubbles cannot rise at all or only rise inadequately through the cold surface layer or strong foam formation occurs.
2. When using an additional burner heating, the sulfur-containing burner exhaust gases condensate on the cooled skull fingers and lead to corrosion of the copper as a result of the formation of sulfuric acid. This drastically reduces the service life of the skull crucible.
3. In the case of aggressive glasses, corrosion of the water-cooled copper components in the upper furnace chamber can occur. As a result of direct flaking of the corroded cooling finger surface or by conveyance via the gas phase, the metallic impurities reach the glass melt and lead to discolorations of the melt.
The object of the present invention is to provide a high-frequency heated skull crucible without a ceramic inner crucible for heating glass melts to temperatures of up to 3000xc2x0 C., preferably up to 2600xc2x0 C., and the glass surface to temperatures of up to 2600xc2x0 C., preferably up to 2400xc2x0 C., and in which the metallic cooling fingers are protected against corrosion by condensed combustion gases or evaporation products.
This object is achieved by the features of claim 1.
The following is achieved in detail by the invention: The cooling fingers are completely covered by the glass melt on the side facing the glass melt. They are thus protected against the exhaust gases or evaporation products from the hot glass surface.
This is achieved in such a way that the metallic cooling fingers converge in the upper crucible zone, but underneath the glass surface, from the perpendicular into the horizontal. This convergence can be made gradually or the cooling pipes are bent by 90xc2x0. The bending of the cooling pipes into the horizontal leads to a cooled collar a short distance underneath the melt surface. The temperature of the glass melt decreases outwardly in the zone of the collar. The glass melt can be cooled off in the edge zone of the collar to such an extent that a ring made of ceramic refractory material can be placed on the edge of the collar. The temperature in the edge zone can be set in the edge zone via the collar diameter and the glass level, so that even at very high melt temperatures in the core zone the glass can be cooled off in the outer zone and can be held by the refractory edge.
Corrosion problems on the metallic cooling fingers are thus avoided. The service life of the metal pipes and thus of the crucible per se is increased by several times.
Furthermore, the glass surface is screened from the cooling fingers by the melt per se. The melt prevents that the upper furnace chamber is cooled in an undesirable way by the cooling fingers. Higher temperatures can thus be achieved in the upper furnace chamber in a controlled way, so that higher temperatures are also obtained in the surface layer of the melt. This is particularly advantageous during the refining. It is thus possible to either omit the addition of refining agents or the refining process can be performed in a shorter period.
The mushroom-like crucible shape in accordance with the invention is not only advantageous during the refining but already during the melt-down process. Because the surface assumes higher temperatures than in conventional crucibles, there is a more rapid melt-down of the glass batch. The throughput is thus increased as compared with known crucibles. A further advantage of the invention is that no corrosion products of the cooling finger can reach the glass melt.
The invention allows meeting all requirements for technical as well as optical glasses, which especially includes the demand for favorable transparency, whereby the glasses must be free from any bubbles.
During the refining with a mushroom-type crucible according to the invention, the glass is liberated from physically and chemically bound gases. The refining process is supported during conventional glass melting by refining agents such as N2SO4, As2O3, Sb2O3, or NaCl. Said refining agents decompose or evaporate at refining temperature and form bubbles into which the residual gases from the melt can diffuse. The refining bubbles must be sufficiently large in order to rise to the surface in the glass melt and burst there within economically viable times. The rising speed of the bubbles depends both on the bubble size and the viscosity of the glass. In the case of a temperature increase from 1600xc2x0 C. to 2400xc2x0 C., the rising speed increases approximately by a factor of 100, i.e. a bubble with a diameter of 0.1 mm rises at 2400xc2x0 C. as fast as a bubble of 1 mm at 1600xc2x0 C.
By increasing the refining temperature, the physical and chemical solubility is decreased in most gases and thus the high-temperature refining is additionally supported.
The high-temperature refining offers the possibility either to radically reduce the refining time or to omit the addition of refining agents for producing large refining bubbles. The precondition is, however, that the rising gas can reach the glass surface and the bubbles disposed at the surface will burst and no foam is formed.
A particularly decisive advantage is thus the extraordinarily high temperature which can be achieved with the invention.
The heating of the mushroom-type crucible in accordance with the invention is carried out substantially by irradiation with high-frequency energy in the crucible zone below the collar. The melt surface is considerably hotter in the upper furnace chamber due to thermal insulation than in the simple known cylindrical skull crucibles.
In the mushroom-type crucible in accordance with the invention, the melt surface can be additionally heated by a gas burner or radiation heating. The exhaust gases from the burner cannot condensate on cold components in this arrangement. Instead, they are led off from the crucible zone via an exhaust gas opening. The same applies to evaporation products from the hot glass surface. As a result, there are no longer any corrosion problems on the metallic cooling fingers and the mushroom-type crucibles have a virtually unlimited service life.
The increase of the temperature in the melt surface by improved insulation of the upper furnace chamber or by additional heating with gas burners or radiation heating also produces an improved coupling of the high frequency into this zone, because the hotter glass surface layers have a higher conductivity than cold ones. A self-amplifying effect is thus obtained.
Improved results could also be achieved for the refining due to the hot melt surface, because a hot glass surface is a precondition for an effective bubble emission from the melt. Although the glass surface shows a descending temperature gradient towards the edges, the bubbles which are produced in the vertical part of the crucible and rise perpendicularly meet a hot glass surface. A rapid rise of bubbles and thus rapid bursting of the bubbles is ensured.